Sleep should last until the machine is woken up with manual intervention.

It's hard to say exactly what is causing your Mac to wake back up, since you haven't mentioned what model you have and what peripherals you use with it, which can often wake the machine up, depending on how they're configured.

Liquid and computers don't mix. It might seem simple, but we see an incredible amount of people post here about spills. Keep drinks and other liquids away from your expensive electronics!